It also turns out that the server has Activity dating back to 9 Oct 2012. The sqbdata process is responsible for the long running query, hence the 6 full months of activity may be the underlying issue, not the client or server dat or sdf files...

Maintenance plan for History Cleanup is now configured, executed, and we're down to 30-days.

No need to remove the file after I whacked the extraneous backup history. All clients now refresh the activity history just fine.

I deployed the server bits to three more production servers that have only 30-day history retention and had no issues at all. Their backup schedules are very similar to the 6-month-retention machine, BTW.

The fact that SQL Monitor kicked out a Long Running Query Alert for sqbdata as the underlying process tells me that the issue lies in THAT process and that the volume of data gave it a two hour twenty five minute headache!